Miniature pressure sensors or transducers mounted at the distal end of catheters are commonly used to measure internal blood pressure in patients. The pressure sensor and catheter are inserted into the subject and positioned at a measurement site of interest. Such invasive pressure sensors provide high quality signals which permit very accurate determinations of the location and extent of such problems as heart valve malformations and malfunctions.
Examples of presently known invasive pressure sensors can be found in U.S. Pat. No. 3,088,323 to Welkowitz et al, U.S. Pat. No. 3,550,583 to Chiku et al, U.S. Pat. No. 3,710,781 to Hutchins, IV et al, U.S. Pat. No. 3,724,274 and U.S. Pat. No. 3,748,623 to Millar, U.S. Pat. No. 4,023,562 to Hynecek et al, U.S. Pat. No. 4,191,193 to Seo, U.S. Pat. No. 4,274,423 to Mizuno et al, and U.S. Pat. No. 4,554,927 to Fussell. Several of the foregoing patents show the use of temperature compensation features in association with the pressure sensors, which adjust the ultimate pressure readings to correct for inaccuracies that might otherwise be caused by temperature variations.
The present pressure sensor invention represents an improvement over the temperature-compensated and other types of pressure sensors disclosed in the aforementioned U.S. patents. It will be described in connection with a temperature-compensated pressure sensor of the type disclosed in U.S. Pat. No. 4,554,927 to Fussell, the disclosure of which patent is incorporated herein by reference. The Fussell patent is assigned to the assignee of the present invention.
The typical prior art pressure sensor includes a housing having an internal chamber. It also includes a pressure responsive element having inner and outer portions exposed respectively to atmospheric pressure conditions in the internal chamber and to ambient pressure conditions outside of the housing. In addition, a piezoresistive element is positioned in the internal chamber, and means are provided to couple the pressure responsive element to the piezoresistive element so that pressure changes outside of the housing are reflected by resistance changes in the piezoresistive element. Typically, the pressure sensor also includes a reference piezoresistance element which provides some level of temperature compensation, and compensating electrical circuitry is provided to accommodate differences in thermal coefficients of expansion which may affect the pressure responsive and temperature responsive piezoresistive elements differently.
As a result of aging characteristics of the pressure responsive element, the piezoresistive elements, circuit elements, and other construction materials, there generally are drift characteristics associated with the zero output and the gain characteristics of the pressure sensor and its allied circuitry. Accordingly, during use, it is often necessary to recalibrate the sensor to be certain that any drift that may have occurred does not result in erroneous readings. In order to recalibrate the sensor, it is necessary to establish a zero differential pressure to check for zero drift and then to apply a known, non-zero, pressure differential to check for gain drift. Such calibration is particularly difficult for devices that are capable of making in vivo measurements. The reason for this problem is that there is no way of accessing the portion of the pressure sensor which is exposed to the measurement site (i.e., the exposed surface of the pressure responsive element).
It is, therefore, a primary object of the present invention to provide an improved pressure sensor which permits calibration of its zero offset due to drift while the pressure sensor is in an in vivo location.
It is another object of the present invention to provide a method for checking the calibration of a pressure sensor while it is located in an in vivo measurement site.
An additional object of the present invention is to provide a method of recalibrating a pressure sensor while it is located in an in vivo measurement site.
Further objects and advantages of this invention will become apparent as the following description proceeds.